1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Moments of inertia

  1. Apr 11, 2005 #1
    Hi, can anyone help me understand how to find the moments of inertia for the following:
    1- A triangular lamina (isosceles) of mass M, base 2B and height H. about line of symmetry.
    2- A uniform lamina of mass M, bounded by the curve with equation y²=4ax and the line x = 4a about the x-axis.

    For (1), I managed to get the answer, but I’m not sure if my way is right: it was finding M.I. of a rectangle base 2B, height H, mass 2M about the line of symmetry through the base, and divide it by 2 (as the triangle is the half of the rectangle!) I just don’t know if this method is right or wrong, or whether there is another method that I should had used. Those questions are from the book, and the book doesn’t explain how to find M.I. for such shapes, it only shows: rod, hoop, and discs. But not a triangle or curves!

    Thank you very much,
    Help is appreciated
  2. jcsd
  3. Apr 11, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    You are proceeding along a wrong track here, I'm afraid.
    Have you learnt that in general, an object's moment of inertia I with respect to an axis is given by the integral:
    where V is the volume of the object (in your 2-D case, an area), r the distance of a mass point within the object to the axis, and dm the (infinitesemal) mass of the mass point?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook